Good idea, I'm possibly looking at cork too if I can find a good cheap source. I think making out of something with ready made detail is the way to do it.
I've done some more. First a couple more of the "cogs"...

Then a hacked lump o' foam...

And my fave, slate chips...

The slate ones were the fastest by far and with all that lovely detail which required no sculpting. Basically glued them onto a base, added some basetex, painted and done. The others are foam, not only had to be shaped but sealed too, takes too long I think.